GPR, or ground-penetrating RADAR (where RADAR is “RAdio Detection And Ranging), is a technology used to assess the composition and location of heterogeneous materials. GPR uses common radio frequencies and is particularly useful in that it is both non-destructive and non-ionizing. In fact, GPR uses frequencies similar to a cellular phone, but at far lower power levels. Common applications include locating the precise position of rebar within a concrete wall/floor, identifying and locating buried objects underground, assessing the quality and uniformity of an asphalt or concrete highway surface, and detecting deterioration on bridge decks. In road surface applications, GPR is used, for example, to detect cracks, fissures, or contamination in any one of the chip seal, pavement layers, gravel base, and so forth. In many roadway applications, a resolution of features of the road surface and internal layer composition of less than one inch (2.54 cm) is desired. Such systems may be mounted on vehicles, passing over the surface while acquiring measurement data. GPR systems are disclosed in more detail in U.S. Pat. No. 5,499,029 to Bashforth, et al., and U.S. Pat. No. 5,384,715 to Lytton, which are hereby incorporated by reference.
There are two common types of GPR for road/bridge surface measurement: Ground-coupled and air-launched. Each has drawbacks. Ground-coupled systems rely upon an antenna that is placed very close to the roadway/surface. At high speeds, the acquired data becomes unusable or the resolution is greatly diminished due to vibrations or even damage to the ground-coupled antenna itself. In fact, the maximum feasible speed of use known in the art is no more than about 15 kph (10 mph), which is highly inefficient, when, for example, there is a need to detect flaws in a long stretch of a six-lane highway with regular traffic flow.
Air-launched systems, on the other hand, utilize antennas aimed at the surface from a height of 30-50 cm (12-20 inches). Unfortunately, air-launched systems, while allowing higher speeds of travel (e.g., at the 105 KPH/65 MPH speed limit of many highways) with less vibration than ground-coupled systems, tend to return very low resolution or unusable data due to interference from sources such as FM (frequency modulation) radio and television (TV) broadcasting. For detailed project-level work, such as bridge deck investigations, if near a major TV transmitter or cellular/PCS transmitter, use of the air-launched antenna becomes impossible. For such cases, alternate data collection methods, using ground-coupled antennas, are currently necessary.
While analog filters are known in the art and may be used to remove unwanted frequencies (e.g., FM or TV broadcasts) from an air-launched system, all known implementable analog filter systems (filters that operate on voltages, currents, or mechanical vibrations) exhibit variations in time delay at different frequencies. A UWB transmission is one in which the bandwidth extends at least the smaller of 500 MHz or 20% of the center frequency. Such a transmission becomes distorted and blurred after passing through such an analog filter. It becomes unknown which changes are due to road surface variations and which are due to the effects of the filter, and the pulse is unrecoverable. Alternatively, digital filters that operate either in software or in the digital hardware of a RADAR system are capable of performing filtering without introducing any kind of ‘blurring,’ but cannot remove the residual effects of overloading that occur in the analog circuitry.
Thus, there remains an unsolved need in the art to provide a GPR system capable of operating at high speeds, such as regular roadway speeds, which provides acceptable data for use in detecting roadway problems.